The present invention relates to a scanner employing a serial scanning head, a printer employing a serial printing head, and a copier employing a serial scanning head and a serial printing head.
A conventional copier of the above type copies an image from a document or other subject copy to a sheet of paper or other printing media as follows. The scanning head makes a transverse scan across the document, obtaining dot data for a certain number of pixel (picture-element) lines. The serial printing head makes a similar transverse scan across the paper, printing the pixels in those pixel lines or a previously scanned group of pixel lines. The document and paper are then advanced in the longitudinal direction, perpendicular to the transverse scanning direction, into position to scan and print the next group of pixel lines. Alternatively, the document and paper remain stationary and the scanning head and printing head are advanced in the longitudinal direction. The next group of pixel lines is scanned and printed after the longitudinal motion halts.
A problem that occurs in this type of copier is that the copy tends to have a banded appearance. If the paper or printing head does not move by exactly the right amount at each longitudinal advance, adjacent groups of pixel lines may overlap, or spaces may appear between the adjacent groups of pixel lines, so that transverse dark or light seams appear in the printed copy. These seams are readily visible and detract greatly from the appearance of the copy.
A known method of reducing the banded appearance causes adjacent groups of printed pixel lines to overlap intentionally, and prints only half of the pixels in the overlapping region during each transverse printing scan. If the overlapping region consists of one pixel line, for example, during each transverse printing scan, the even numbered pixels in the first pixel line and the odd-numbered pixels in the last pixel line can be printed. If the overlapping region is more than one pixel line wide, it can be printed in a checkerboard fashion.
This scheme does not entirely eliminate the banded appearance. Particularly in copies of dark images, misalignment of pixels in the overlapping regions creates a regular pattern of density variations and other printing defects that is still quite visible.
Similar defects appear when a computer-generated image, or an image read from an image file, is printed by a printer having a serial scanning head. Seams can also appear in images scanned by an image scanner having a serial scanning head, regardless of how the image is reproduced, if there are irregularities in the longitudinal motion between transverse scans.